As methods for adjusting the focus of an optical microscope when observing the inside of an observation sample, the following methods are known. In a method described in Patent Literature 1, a system detects the interface of a sample (e.g., the lower surface of a container) and focuses on the detected interface. The system shifts the focal position to the inside of the sample by moving an objective lens in a Z-direction (in the direction of the optical axis of the objective lens) by a predetermined amount (a fixed distance) with respect the interface. The system then acquires many images at predetermined intervals in the Z-direction inside the sample and determines the most appropriate focal position on the basis of the acquired many images.
In a Method described in Patent Literature 2, a system detects the interface of a sample (e slide glass or cover glass) and focuses on the detected interface. The system then moves at least one of a stage and an objective lens in the Z-direction by a predetermined amount. The system then captures images of the sample at predetermined measurement time intervals while moving at least one of the stage and objective lens in the Z-direction (to the inside of the sample). In this way, time lapse observation is performed using the method described in Patent Literature 2.